Re: [PATCH v1 03/11] io_uring/kbuf: add support for kernel-managed buffer rings

[Joanne Koong <joannelkoong@gmail.com>]

On Thu, Feb 12, 2026 at 11:27 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Thu, Feb 12, 2026 at 09:29:31AM -0800, Joanne Koong wrote:
> > > > I'm arguing exactly against this.  For my use case I need a setup
> > > > where the kernel controls the allocation fully and guarantees user
> > > > processes can only read the memory but never write to it.  I'd love
> >
> > By "control the allocation fully" do you mean for your use case, the
> > allocation/setup isn't triggered by userspace but is initiated by the
> > kernel (eg user never explicitly registers any kbuf ring, the kernel
> > just uses the kbuf ring data structure internally and users can read
> > the buffer contents)? If userspace initiates the setup of the kbuf
> > ring, going through IORING_REGISTER_MEM_REGION would be semantically
> > the same, except the buffer allocation by the kernel now happens
> > before the ring is created and then later populated into the ring.
> > userspace would still need to make an mmap call to the region and the
> > kernel could enforce that as read-only. But if userspace doesn't
> > initiate the setup, then going through IORING_REGISTER_MEM_REGION gets
> > uglier.
>
> The idea is that the application tells the kernel that it wants to use
> a fixed buffer pool for reads.  Right now the application does this
> using io_uring_register_buffers().  The problem with that is that
> io_uring_register_buffers ends up just doing a pin of the memory,
> but the application or, in case of shared memory, someone else could
> still modify the memory.  If the underlying file system or storage
> device needs verify checksums, or worse rebuild data from parity
> (or uncompress), it needs to ensure that the memory it is operating
> on can't be modified by someone else.

(resending because I hit reply instead of reply-all)

I think we have the exact same use case, except your buffers need to
be read-only. I think your use case benefits from the same memory wins
we'll get with incremental buffer consumption, which is the primary
reason fuse is using a bufring instead of fixed buffers.

>
> So I've been thinking of a version of io_uring_register_buffers where
> the buffers are not provided by the application, but instead by the
> kernel and mapped into the application address space read-only for
> a while, and I thought I could implement this on top of your series,
> but I have to admit I haven't really looked into the details all
> that much.

I think you can and it'll be very easy to do so. All that would be
needed is to pass in a read-only flag from the userspace side when it
registers the bufring, and then when userspace makes the mmap call to
the bufring, the kernel checks if that read-only flag is set on the
bufring and if so returns a read-only mapping. I'm happy to add that
patch to this series if that would make things easier for you. The
io_uring_register_buffers() api registers fixed buffers (which have to
be user-allocated memory) so you would need to go through the
io_uring_register_buf_ring() api once kmbufs are squashed into the
pbuf interface.

With going through IORING_MEM_REGION, this would work for your use
case as well. The user would have to register the mem region with
io_uring_register_region() and pass in a read-only flag, and then the
kernel will allocate the memory region. Then userspace would mmap the
memory region and on the kernel side, it would set the mapping to be
read-only. When the kmbufring then gets registered, the buffers in it
will be empty. The filesystem will then have to populate the buffers
in it from the mem region that was previously registered.

Thanks,
Joanne

>
> >
> > To be completely honest, the more I look at this the more this feels
> > like overkill / over-engineered to me. I get that now the user can do
> > the PMD optimization, but does that actually lead to noticeable
> > performance benefits? It seems especially confusing with them going
> > through the same pbuf ring interface but having totally different
> > expectations.
>
> Yes.  The PMD mapping also is not that relevant.  Both AMD (implicit)
> and ARM (explicit) have optimizations for contiguous PTEs that are
> almost as valuable.
>
> > What about adding a straightforward kmbuf ring that goes through the
> > pbuf interface (eg the design in this patchset) and then in the future
> > adding an interface for pbuf rings (both kernel-managed and
> > non-kernel-managed) to go through IORING_REGISTERED_MEM_REGIONS if
> > users end up needing/wanting to have their rings populated that way?
>
> That feels much simpler to me as well.
>